A typical vacuum fluorescent display device comprises a transparent evacuated envelope containing a plurality of anodes arranged in a pattern of desired light emission, each anode being coated with a fluorescent layer for emitting light when excited, a heated filament serving as a source of electrons, and control grids located between the filament and the anodes for determining which anodes can be excited by the electrons. When the anodes and the control grids are at a high voltage and the filament is at a lower voltage the electrons can excite the fluorescent layer on the anodes to cause light emission from the anodes. The brightness of the emitted light depends very strongly on the voltage between the filament and anode. When dc heating current is applied to the filament, a voltage drop occurs along the filament so that at one end of the filament the anode to filament voltage is somewhat less than at the other end so that a difference of brightness occurs across the display. The use of alternating current in the filament overcomes this brightness difference provided the frequency is high enough that no perceptible flicker occurs. As is well known, the eye responds relatively slowly to light impulses so that repetitive images appear as one and brightness variations in the images are averaged over time, provided that the images occur at a sufficiently rapid rate, called the critical flicker fusion frequency. When brightness variations are time averaged over a critical fusion period the differences are imperceptible. The critical frequency is regarded to be in the range of 16 to 30 Hz in the case of a stationary display; however, where the display may rapidly move relative to the observer, such as may occur in a moving automotive vehicle, a stroboscopic effect occurs so that a higher rate, say 100 to 130 Hz is required to achieve the appearance of a continuous light source. The application of AC driving potential to a filament can cause acoustic affects or singing due to the filament vibration. Thus the frequency of the filament current must be high enough to minimize the the acoustic affects. This requires that the frequency should be above 20 kHz. 0n the other hand, the frequency should be low enough to minimize radio frequency interference and inductive problems.
A number of mechanisms are available for controlling the brightness of a display. A preferred method is to use a variable duty cycle for anode energization so that maximum brightness is obtained when an anode is illuminated 100% of the time or, in the case of a multiplex system, when it is illuminated for 100% of its allotted time slot. In such cases when the filament current frequency is sufficiently high many filament cycles occur during each ON cycle of the anode and the non-uniformity of brightness discussed above does not present any problem. It is often required, however, that a dimming ratio in excess of 200 is required in order to obtain satisfactory extremes of brightness. In the case of very low brightness when the duty cycle is, for example, one percent or less of the allotted anode ON time the actual ON time may be but a fraction of a filament current cycle. In that circumstance, the display non-uniformity or display flicker is a significant problem. Flicker can occur because at certain duty cycles many successive anode ON times can occur for a given anode when the filament voltage is at a low state and then later on a similar succession of anode 0N times can occur when the filament voltage is at a high state so that time averaging of the anode brightness over its entire brightness fluctuation range does not occur within the critical flicker fusion period.
U.S. Pat. No. 4,495,445 reveals a brightness control for vacuum fluorescent display. It recognizes the brightness non-uniformity problems, especially at low brightness levels. The control circuit operates at a filament frequency of 60 Hz and the anode ON time is, at most, one period of the filament current. With that restriction, a large dimming ratio is not possible except at low frequencies. Display brightness is controlled by varying the duty cycle of the anode, utilizing a duty cycle symmetrical about the zero crossing of the filament voltage, and having ON times close to the zero crossing. The non-uniformity at low brightness is minimized since the potential variation along the length of the filament is minimized in the vicinity of the zero crossing. That control circuit is particularly designed for operation on sine wave control signals and it does not teach a control method useful with a high frequency digital logic control or using square wave control pulses. Moreover, such a control circuit is not amenable to both high frequency operation and high dimming ratios.